1. Technical Field of the Invention
The present invention relates to an optical beam scanning apparatus and an image forming apparatus equipped with the optical beam scanning apparatus, and more particularly, to an optical beam scanning apparatus which is capable of adjusting deviation of a recording position using a horizontal synchronization sensor, and an image forming apparatus equipped with the optical beam scanning apparatus.
2. Description of the Related Art
Image forming apparatuses employing an electrophotographic method, such as a laser printer, a digital copying machine, a laser facsimile machine and so on, each have an optical beam scanning apparatus for forming an electrostatic latent image on a photoconductive drum by irradiating and scanning a surface of the photoconductive drum with a laser beam (light beam).
In recent years, a tandem color apparatus has been proposed in addition to a monochrome apparatus equipped with a scanning optical system using a single light source, and in addition, a method for use in the tandem color apparatus has been proposed, which increases the number of laser beams to be scanned one time using a plurality of light sources (laser diodes) arranged in a single laser unit for the purpose of increasing the scan speed on a surface of a photoconductive drum (multi-beam method). In the multi-beam method, a plurality of beams for each of color components (for example, yellow, magenta, cyan and black) emitted from each light source are processed to be combined into a single integrated beam in an optical system before deflection, and then the single integrated beam is incident on a polygon mirror. The polygon mirror deflects the incident beam which in turn passes through an fθ lens constituting an optical system after deflection to be separated into beams for respective color components to be irradiated on respective photoconductive drums corresponding to the respective color components.
For an optical system using a light source (LD array) having a plurality of emission points (optical system using a multi-beam method), if a beam-to-beam pitch in a sub-scanning direction on an imaging plane (plane in which a laser beam (light flux) is actually imaged) is set to be a desired value (for example, about 42 μm for 600 dpi and about 21 μm for 1200 dpi), since the emission points have a required distance (deviation) therebetween in a main scanning direction without being arranged in a row in the sub-scanning direction in view of magnification of the optical system, there is a need to adjust a recording position. In the multi-beam method using the light source having the plurality of emission points, as recording timings on image planes of photoconductive drums are deviated from each other, positional deviation of the laser beams scanned on the photoconductive drums occurs in the main scanning direction, which may result in deterioration of image quality due to the positional deviation of the laser beams.
In the prior art, for example, in a scanning optical system using a multi-beam method, a portion of light flux reflected on a deflection surface of a polygon mirror is detected by a horizontal synchronization sensor and is used as a horizontal synchronization adjusting signal in order to align a recording position of lead-off laser beam and respective recording positions corresponding to respective laser beams located with required distance in the main scanning direction from thus recording position (that is, in order to take a horizontal synchronization). This horizontal synchronization sensor is used to correct the recording position at regular intervals.
However, the precondition for precise alignment of the recording position of the scanning optical system is that an edge line direction of a light receiving surface of the horizontal synchronization sensor is substantially perpendicular to a main scanning direction in which light fluxes are scanned. If this precondition is not satisfied, it is difficult to properly correct deviation of the light fluxes in the main scanning direction, which occurs on an imaging plane.
To overcome such a problem, JP-A-2002-341272 discloses a technique for preventing deviation of a recording position of a multi-beam laser. In the technique disclosed in JP-A-2002-341272, an integrated unit including BD slits and BD sensors (BD detecting unit) is arranged around an optical axis of a scanning lens in a rotationally adjustable manner.
The following techniques have been known as techniques related to the technique for preventing deviation of the recording position of the multi-beam laser.
According to a technique disclosed in JP-A-2006-215483, by moving position in an optical axial direction of reference light flux detecting means (synchronization detecting sensors), which is arranged at an optical scanning start side and an optical scanning end side, toward a predetermined direction, it is possible to reduce positional deviation in a main scanning direction between two laser beams (deviation between the optical scanning start side and the optical scanning end side), which occurs when a light flux incident on an optical deflector (or a scanning optical system) is a “convergent light flux”, thereby preventing image quality from being deteriorated.
According to a technique disclosed in JP-A-2002-107645, delay time from detection of a synchronization detecting signal to write initiation may be adjusted either in an electrical manner or in a mechanical manner such as adjusting an angle of a mirror to direct a light flux to a light receiving means.
According to a technique disclosed in JP-A-2000-255096, by adjusting an attachment position of a BD unit in advance such that scan light for obtaining a reference signal of image write is incident on the center of an effective detection portion of a BD unit, it is possible to improve compatibility of a scanner unit.
According to a technique disclosed in JP-A-2002-341271, a BD sensor and a BD slit plate for passing a laser beam to the BD sensor are accommodated in an optical box, and a BD slit provided in the BD slit plate is formed with a slit shape inclined at a predetermined angle θ (0°<θ<90°) with respect to a scan direction of a scan lens, thereby achieving simplification of BD recording timing adjustment.
According to a technique disclosed in JP-A-2002-357778, an optical beam scanning apparatus in which an optical box attached with a light source part, a deflector, an optical part, a horizontal synchronization detecting means and a reflecting member is attached to an optical bench by means of a plurality of couplers has a support member provided in a shakable manner with respect to the optical box for supporting and fixing the reflecting member on the optical box by adjusting its position relative to the optical box.
According to a technique disclosed in JP-A-2004-333556, for a small-sized scanning optical apparatus to direct a light flux to a BD sensor via a BD lens without passing through a scan lens, recording adjustment of scan lines may be made by moving the BD lens in a main scanning direction.
However, in the technique disclosed in JP-A-2002-341272, since there exists no reference required for rotational adjustment of the horizontal synchronization sensor, there is a need to make the rotational adjustment while seeing deviation of beam spots of light fluxes being actually scanned. However, since a group of beam spots has no vertical reference to emission points, there is a problem of difficulty in producing verticality of the horizontal synchronization sensor on the basis of a group of actual beam spots. In addition, there is a problem of difficulty in determining positions of the horizontal synchronization sensor and the slits.